1. Field of the Invention
The present invention relates to an apparatus for compensating for the chromatic dispersion and the dispersion slope generated in optical signals of respective wavelengths which are contained in a wavelength division multiplexed light transmitted through an optical fiber transmission path, and in particular, to an apparatus capable of variably compensating for the chromatic dispersion and the dispersion slope independent of each other.
2. Description of the Related Art
In an optical transmission system, it is necessary to perform the chromatic dispersion compensation, in order to suppress the wavelength deterioration which occurs due to a difference between optical fiber propagation times of respective wavelength components of modulated optical signals. As one of conventional chromatic dispersion compensators, there has been known, for example, a chromatic dispersion compensator configured by utilizing a so-called virtually imaged phased array (VIPA) for demultiplexing a signal light into a plurality of optical beams that can be distinguished spatially according to wavelengths (for example, a plurality of optical beams that travel to different directions) (refer to Japanese National Publication No. 2000-511655).
Here, there will be briefly described a conventional VIPA-type chromatic dispersion compensator.
FIG. 13 is a perspective view showing a configuration example of the conventional VIPA-type chromatic dispersion compensator. Further, FIG. 14 is a top view of the configuration example of FIG. 13.
As shown in each figure, in the conventional VIPA-type chromatic dispersion compensator, a light emitted from one end of an optical fiber 130 via an optical circulator 120 is converted into a parallel light by a collimate lens 140 and, then, condensed on one segment by a line focusing lens 150 and passes through a radiation window 116 of a VIPA plate 110 to be incident between opposed parallel planes. The incident light on the VIPA plate 110 is multiple-reflected repeatedly, for example, between a reflective multilayer film 112 formed on one plane of the VIPA plate 110 and having the reflectance lower than 100% and a reflective multilayer film 114 formed on the other plane and having the reflectance of substantially 100%. At this time, every time the incident light is reflected on the surface of the reflective multilayer film 112, a few % of the light is transmitted through the surface to be emitted outside the VIPA plate 110.
The lights transmitted through the VIPA plate 110 interfere mutually and form a plurality of optical beams, traveling directions of which are different from each other, according to wavelengths. As a result, if each of the optical beams is condensed to one point by a convergent lens 160, each condensed position moves on a straight line according to variation of the wavelengths. By disposing, for example, a three-dimensional mirror 170 on the straight line, the lights that have been emitted from the VIPA plate 110 and condensed by the convergent lens 160 are reflected at different positions on the three-dimensional mirror 170 according to respective wavelengths to be returned to the VIPA plate 110. Since the lights reflected on the three-dimensional mirror 170 travel to different directions according to wavelengths, optical paths thereof are deviated when they are returned to the VIPA plate 110. The optical path deviation amounts are changed according to wavelengths, so that different wavelength components are propagated for different distances and, therefore, the chromatic dispersion of the input light is compensated.
In consideration of a model as shown in FIG. 15, for example, behavior of the light that is multiple-reflected by the VIPA plate 110 as described above is similar to that in an Echelon grating that is well-known as a step-wise diffraction grating. Therefore, the VIPA plate 110 can be considered as a virtual diffraction grating. Further, in consideration of an interference condition in the VIPA plate 110, as shown on the right side in FIG. 15, the emitted light interferes under a condition in which, with an optical axis thereof as a reference, a shorter wavelength is above the optical axis and a longer wavelength is below the optical axis. Therefore, among a plurality of optical signals of respective wavelengths, optical signals on the shorter wavelength side are output above the optical axis and optical signals on the longer wavelength side are output below the optical axis. Such a conventional VIPA-type chromatic dispersion compensator has advantages in that the chromatic dispersion can be compensated over a wide range and also the wavelength (transmitted wavelength) of the optical signal to be compensated can be varied.
Further, in a system for transmitting a wavelength division multiplexed (WDM) light containing a plurality of optical signals having different wavelengths, since it is necessary to perform the appropriate chromatic dispersion compensation on the optical signals of respective wavelengths, there is a case where also the wavelength dependence of chromatic dispersion, which is called the dispersion slope, is required to be compensated. Combinations of the chromatic dispersion and the dispersion slope, which are to be compensated, exist in countless numbers, since there are countless combinations of the type of optical fiber and the length of optical fiber transmission path. Therefore, there is demanded an apparatus capable of variably compensating the chromatic dispersion and the dispersion slope independent of each other.
As a conventional technology for variably compensating for the chromatic dispersion and the dispersion slope independent of each other, as shown in FIG. 16 for example, there has been proposed an apparatus in which, in the above described VIPA-type chromatic dispersion compensator, there is disposed means 180 for generating the optical path deviation between the lights of respective wavelengths angularly dispersed by the VIPA plate 110 in parallel according to the wavelengths, in a direction vertical to an angular dispersion direction of the lights, so that the chromatic dispersion and the dispersion slope, which are to be given to the optical signals of respective wavelengths contained in the WDM light, can be controlled independent of each other (refer to Japanese Unexamined Patent Publication No. 2002-258207).
However, in the conventional chromatic dispersion and dispersion slope compensating apparatus utilizing the VIPA described above, there is a problem of requiring a complicated control mechanism. Namely, the conventional apparatus needs to be disposed with a mechanism controlling a position of the three-dimensional mirror 170 in order to compensate for the chromatic dispersion, and the means 180 for generating the optical path deviation in the direction vertical to the angular dispersion direction of the VIPA in order to variably compensate for the dispersion slope, to be specific, a parallel plate composed of a transparent material having the refractive index chromatic dispersion or two prisms, or a mechanism controlling two diffraction gratings. If the optical path deviation amount is varied by controlling the means 180 such as the parallel plate or the like, in order to vary a compensation amount of the chromatic dispersion, the optical paths themselves of the respective wavelengths toward the three-dimensional mirror 170 are also deviated overall and a compensation amount of the chromatic dispersion is changed. Therefore, these mechanisms need to be controlled in association with each other. However, for operating the three-dimensional mirror 170 in association with the means 180 to move them for an infinitesimal distance, a complicated adjusting mechanism is needed. But, the designing and the manufacturing of such an adjusting mechanism are not easy and an enormous cost is required. Therefore, there has been a problem in that the conventional apparatus is actually hard to be in practical use.